A thin film transistor liquid crystal display (TFT LCD) uses a thin film transistor as a switching device and the electrical-optical effect of liquid crystal molecules to display data visually. A display is typically composed of a TFT substrate in which a plurality of liquid crystal pixels having a TFT and a pixel electrode are formed, a substrate where a common electrode is formed, and liquid crystal material sealed therebetween, as will be understood by those skilled in the art.
Methods for achieving gray scale representation in TFT LCDs have been achieved based on the electrical-optical response curve of the liquid crystal material. The contrast ratio of TFT LCDs vary in accordance with the viewing angle. Also, a viewing angle dependence of the contrast ratio is varied regarding optical transmission. This dependance of the viewing angle is significant in a twisted nematic type of LCD as will be understood by those skilled in the art. As a result, gray scale errors can be introduced when TFT LCDs are viewed off-normal. This gray scale error increases with increases in the viewing angle, to thereby limit the allowable maximum viewing angle. Also, the dependence of the angle of viewing on the characteristics of the liquid crystal display is typically more severe in a vertical direction than in a horizontal direction.
Much work has been done for improving the viewing angle characteristics of TFT LCDs. For example, a TN cell using optical compensation films, a TN cell using subpixels, and a multi-domain TN cell have been proposed. However, since characteristics of asymmetrical visibility and gradation inversion remain, the optical compensation method using an optical compensation film has little effect in enhancing the angle of the viewing. The multi-domain TN cell, such as a dual-domain TN cell, requires an increase in the number of fabrication steps because it requires a plurality of photolithography processes and a plurality of rubbing processes. However, these additional steps can cause a reduction in yield. The TN cell using subpixels causes the problems that the open area ratio of the pixel is lowered and the number of fabrication steps therefor is increased.
FIGS. 1-3 show various dual-domain TN cells. A complementary TN cell structure shown in FIG. 1 has an alignment layer having a low pre-tilt angle formed on an upper substrate and an alignment layer having a high pre-tilt angle formed on a lower substrate, respectively. In addition, the alignment layer formed on the lower substrate has a different direction of alignment by domains. Referring to FIG. 2, a polyimide film is formed on the lower substrate and undergoes a rubbing process in a first alignment direction. Then, a photoresist pattern is formed. This pattern is for dividing the cell into two domains. Then, another rubbing process is performed in a second alignment direction which is opposite to the first alignment direction. Thus, the part under the photoresist pattern doesn't undergo the second rubbing process, while the remnant part undergoes the second rubbing process. Subsequently, the photoresist pattern is removed.
In a TN cell structure shown in FIG. 3, two different alignment layers are sequentially formed on both upper and lower substrates. Here, the first alignment layers have low pre-tilt angle and the second alignment layers have high pre-tilt angle. The second alignment layers are also patterned by a photolithography method and may be made of an inorganic material. Thus, the rubbing process for the second alignment layer of high pre-tilt angle doesn't affect the first alignment layer.
FIGS. 4A, 4B and 5 show the conventional TN cell using subpixels. Referring to FIG. 4A, a liquid crystal pixel is divided into a plurality of sub-pixels, i.e., sub-pixel 1, sub-pixel 2 and sub-pixel 3, with the sub-pixels each having different liquid crystal capacitances C.sub.LC1, C.sub.LC2 and C.sub.LC3, respectively. FIG. 4B shows the equivalent circuit of FIG. 4A. Each cell also has two different control capacitors C.sub.C2 and C.sub.C3. The control capacitors C.sub.C2 and C.sub.C3 of the cell, which are selectively connected to three liquid crystal capacitors C.sub.LC1, C.sub.LC2 and C.sub.LC3, serve as a voltage divider and supply a control voltage to each of the sub-pixels. Accordingly, even though the voltage applied through a TFT to the pixel electrode is one value, different voltages are applied to the sub-pixel liquid crystal capacitors C.sub.LC1, C.sub.LC2 and C.sub.LC3. That is, the voltages applied to the sub-pixels are different.
Thus, the twist angles of the liquid crystal corresponding to the sub-pixels are different. As a result, one liquid crystal cell is composed of three sub-pixels having three kinds of different transmittivities. Here, the transmittivity of a liquid crystal cell is an average value of the three kinds of transmittivities. Since the viewing angle dependence is different according to the transmittivities, the device shown in FIG. 4A can be viewed at relatively large viewing angles.
Referring to FIG. 5, reference numeral 10 indicates a lower substrate formed of glass, reference numeral 12 indicates a gate electrode, reference numeral 14 indicates a gate insulating film, reference numeral 16 indicates a pixel electrode, reference numeral 18 indicates a transparent insulating film and reference character TFT indicates a thin-film switching transistor.
In FIG. 5, three sub-pixel liquid crystal capacitors C.sub.LC1, C.sub.LC2 and C.sub.LC3 represent equivalent capacitances formed in the combination of a common electrode of the upper substrate with electrode layers 16, 16' and 16", respectively. That is, in order to cover the part of the pixel electrode 16, a first transparent insulating layer 18 and a first transparent electrode 16' are formed, and then a second insulating layer 18' and a second transparent electrode 16" are sequentially formed thereon.
However, in order to form the sub-pixel liquid crystal capacitors, the additional steps of stacking transparent insulating layers and patterning the transparent electrodes must be performed. Thus, this device has a low open area ratio and the additional fabrication steps typically lead to a reduction in the yield of the TFT LCD.